This invention relates generally to shell-and-tube heat exchangers having improved tube sheet and front-end head designs.
In industry, heat transfer methods form an important part of almost all chemical processes. One of the most commonly used pieces of heat transfer equipment is the shell-and-tube type heat exchanger. Descriptions of the various types of heat exchangers are summarized in many well known publications, see generally, 1 Perry's Chemical Engineers'Handbook. chap. 11 at 3-21 (Green, 6th ed. 1984), and do not need to be fully described here. Generally, this type of heat exchanger comprises a bundle of tubes and a head having an inlet nozzle in fluid flow communication with an outlet nozzle. The tube bundle is enclosed in a shell that enables one fluid to flow into contact with the tube bundle and to transfer heat from or to another fluid flowing through the tubes in the bundle.
Shell-and-tube heat exchangers may be used in essentially all types of functional services such as condensing, cooling, vaporizing, evaporating, and mere exchanging of heat energy between two different fluids. Furthermore, shell-and-tube exchangers are capable of handling practically any types of chemical compounds including, for example, water, steam, hydrocarbons, acids, and bases. In the design of a shell-and-tube heat exchanger, there are a myriad of mechanical and process factors to take into account in order to generate an economically optimum heat exchanger design. Many of these desirable design factors, however, have off-setting negative results which impose limits on the extent to which a certain design factor may be used. For instance, it is generally desired to maximize the amount heat transferred in an exchanger and, to achieve this, a designer will attempt to increase the heat transfer surface and to maximize the fluid velocity in both the tube-side and the shell-side of the exchanger. But, by increasing the surface area of a heat exchanger and the fluid velocities, the economic cost of exchanger materials escalates and the cost of pumping a fluid through the exchanger increases. Because of these conflicting considerations, a designer must optimize the design of a heat exchanger by comparing the incremental value of the heat recovered to the incremental cost associated with recovering the additional heat energy. The point where the incremental costs and incremental values are equivalent will provide the economic optimum exchanger design.
Another design consideration is the quality and nature of the fluids being handled and their effect on the corrosion, fouling, and scaling of the exchanger surfaces. Fouling is the deposition of material upon the heat transfer surfaces of a heat exchanger. These deposited materials usually have low thermal conductivities which create large thermal resistances thereby lowering the heat transfer coefficient. Having a surface with a high heat transfer coefficient is beneficial in that it provides a greater rate of heat transfer and allows for a more economical heat exchanger equipment design.
One approach to minimizing the rate of fouling of a heat exchanger is to design for high liquid or gas velocities. The disadvantage, however, of designing for high velocities is that the pressure drop across a heat exchanger increases exponentially with increases in velocity which results in increasing fluid pumping costs. Moreover, greater erosion damage of the heat exchanger surfaces is caused by the higher fluid velocities. Because of these negative consequences, heat exchanger design specifications provide for both a minimum fluid velocity flow and a maximum acceptable velocity flow.
When a shell-and-tube type heat exchanger is used as either a vaporizer or as a condenser, either one or both of the fluids passing through the heat exchanger undergo a phase change. Because of this phase change, the volumetric flow rate changes as gas or liquid passes through the heat exchanger. This change in volumetric flow rate results in a change in fluid velocity; and, in the case of a condensing fluid, its velocity will decrease as it passes through the exchanger creating a greater potential for fouling, scaling, or corrosion problems which are associated with low tube-side fluid velocities. In the case where a fluid is being vaporized, its volumetric velocity will increase as it passes through the exchanger creating a greater potential for erosion.
One approach to addressing the problems related to low tube side fluid velocities is to provide for multiple tube passes. This multi-pass type heat exchanger construction provides for an improvement in the heat transfer coefficient through the increase in fluid velocity by decreasing the cross-sectional area of the fluid path. A multi-pass heat exchanger is constructed by building into the head and return ends of a heat exchanger baffles or partitions which direct the fluid through the tubes into their proper relative positions.
The most common multi-pass heat exchanger construction is to arrange for an equal number of tubes per pass; however, if the physical changes in the fluid volumes warrant, a heat exchanger may be designed so that there are an unequal number of tubes per pass. By providing for a heat exchanger with an unequal number of tubes per pass, a heat exchanger can be designed to maintain a relatively even fluid velocity distribution throughout the length of the exchanger tubes even though there is a phase change in the fluid as it passes through the tubes. By controlling the fluid velocity on the tube-side of an exchanger, all of the various design considerations such as fouling, scaling, corrosion, erosion, heat transfer coefficients, and pressure drop can be optimized.
In spite of the various advantages which may accrue from the use of multiple-tube pass exchangers, there are certain disadvantages, which have not been resolved by the art, to using these types of heat exchangers where they are of the type having removable tube bundles. It is sometimes desirable to periodically rotate a heat exchanger tube bundle about its longitudinal axis 180.degree. in order to prolong the useful life of the tubes. This procedure of rotating the exchanger bundle is somewhat analogous to rotating the tires on an automobile in order to prolong the useful life of the tires through a more even distribution of wear. Particularly, where a heat exchanger is being used in a highly corrosive and stressful service, it is important to rotate the tube bundle to allow for a more even distribution of the corrosive, erosive, and other stresses. However, if the heat exchanger is one having equal or unequal numbers of tubes per pass, the tube bundle cannot be rotated as desired because of the unsymmetrical flow pattern.